1. Field of the Invention
This invention relates to the field of ceramics, and particularly to the field of Si.sub.3 N.sub.4 ceramics.
2. Description of the Prior Art
Ceramic compositions produced from silicon nitride and silicon carbide are the two most advanced ceramic materials presently employed. An advantage that silicon nitride has over silicon carbide is in its lower thermal expansion coefficient which makes silicon nitride more thermal shock resistant.
There is a need for making shaped ceramic bodies such as silicon nitride, having high strength and sufficient fracture toughness, that is free of fracture-initiating defects, for use at temperatures and in environments that severely limit the useful life of metals. There is a high probability that the next generation of advanced turbines will contain ceramic components in order to meet increased temperature requirements dictated by increased performance. The temperature range capability of the currently used nickel and cobalt base superalloys is inadequate to meet these requirements. Refractory metal reinforced super-alloys can extend this range, but still the maximum temperature cannot exceed the superalloy melting temperatures, and they are susceptible to elevated temperature environmental corrosion. Limitations of these metal matrix composites include their comparatively high densities, which increase tensile stresses in rotating components, and reactivity of the refractory metal composites with most elevated temperature turbine environments.
Ceramics such as silicon nitride have the advantage of considerably higher temperature capability than the materials of the current turbine components. Ceramics also have comparatively high strength/weight ratios and are virtually inert in most high-temperature environments.
Currently net shape fabrication of ceramics is achieved mainly by injection molding which involves mixing the ceramic powder and sintering aids with an oil soluble plastic binder and injection of the mixture into a mold cavity. The need for defect-free ceramics is the result of the brittle nature of ceramics. Fracture initiation occurs by crack propogation originating at flaws. Agglomeration is a main source of flaws produced during powder processing in preparation for injection molding. The current method of injection molding using oil soluble plastic binders is not compatible with advanced processing methods which involve aqueous solution dispersion for obtaining a homogeneous ceramic powder-sintering aids dispersion and for removing agglomerates and impurities which produce imperfections that lower the strength and toughness of the ceramics.